Iron-catalyzed autoxidation of liposomal cholesterol.

Abstract

The autoxidation of cholesterol in a benzene solution of egg lecithin and Fe (acac)3, giving products variously oxygenated in the steroidal ring B, proceeded with consumption of the unsaturated long-chain fatty acid moieties, particularly C18 : 2, in the lecithin molecule. The reaction showed a marked β-stereoselectivity of epoxidation and was inhibited by a radical scavenger (BHT). Cholesterol was highly susceptible to ferric iron-catalyzed autoxidation within liposomes prepared by using egg lecithin. The oxidative degradation of the unsaturated moieties, C18 : 1, and C18 : 2, in the lecithin led to allylic oxidation as well as the β-stereoselective epoxidation of cholesterol. A radical scavenger inhibited both the degradation of these moieties and the oxygenation of cholesterol. The oxygenation was retarded in liposomes prepared by using saturated dipalmitoyl lecithin, and was dominated by allylic oxidation giving cholesteryl hydroperoxide as the main product. Cholesterol in the liposomes containing egg lecithin was, thus, assumed to be co-oxidized with the unsaturated fatty acid moieties by the radical pathway, when the liposomes were autoxidized in the presence of ferric catalyst.